The problem at the origin of the present invention relates to all the gas-solid reactions involving pulverulent products and thus widely goes beyond the scope of the sole processing of the aforementioned fumes. When heterogeneous reactions are limited by diffusion processes, it is often advisable to operate with very fine products so as to increase the contact surface and to facilitate access of the gases into the particles. On the other hand, the fine particles, i.e. particles with sizes of the order of a micron or of some microns, are very easily carried along by the gas phase. Thus, as soon as they enter the reaction system, the fine particles reach almost instantaneously a displacement velocity that is nearly that of the gas carrying them. As a result, the residence time of the particles in said reaction system is almost identical to that of the gases. This situation can however entail drawbacks when the reaction is slow and might require longer contact times between the solid phase and the gas. On the chemical plane, the consequence of this are poor conversion efficiencies. Although it is possible to increase the conversion efficiency in relation to the gas phase by increasing the solids flow rate, this option is often synonymous with increased operating costs.
Furthermore, it has already been proposed in patent FR-2,669,554 in the name of the applicant to desulfurize combustion gases by fractionating and by remixing several times the gas flow mixed with an absorbent.
In the more specific case of dry processing of fumes from domestic waste incineration plants, the main objective is to collect acid pollutants such as hydrochloric acid by means of an absorbent such as lime. This operation is conducted at low temperature, i.e. between 150 and 400.degree. C., sometimes in the presence of water in order to improve performances somewhat, in reactors of rather simple geometry.
An example of a reactor designed for effluent processing is given in patent FR-2,636,720. This document describes a line provided with a venturi upstream of which means for delivering the absorbent into the gas stream to be processed are arranged. The venturi is intended to provide proper mixing of the powder with the gases. The absorbent collects the acid components during its residence in the line. The gas-absorbent mixture completes its course in a final dust collector which may be an electrostatic precipitator or a bag filter. The residence time of the absorbent in the system is substantially identical to that of the fumes and never exceeds some seconds (2 to 3 seconds for example).
Much longer residence times would be required to complete the reaction under stoichiometric conditions where the Ca/Cl molar ratio is 0.5. This is the reason why a fraction only of the absorbent is used. With current systems, typically 90% of the hydrochloric acid contained in incineration plant fumes can be collected by using absorbent quantities corresponding to a Ca/Cl molar ratio of the order of 1. This means that only just half of the potential of the absorbent is currently used. The used absorbent discharged from the final dust collector thus contains a mixture of calcium chloride and of lime that has not reacted. The process residue is thus not inconsiderable since it represents 40 to 50 kg per ton of domestic waste processed. Disposal problems therefore become increasingly frequent in a context where dumping is prohibited and stabilizing treatments are costly.
It can be noted that, if it is desired to increase the rate of collection of the hydrochlorid acid, the consumption of fresh absorbent has to be significantly increased, which obviously leads to higher residue volumes. To obtain higher acid pollutant collection efficiencies, a solution consists in switching from a dry process to a wet process. In this case, hydrochloric acid collection efficiencies are higher and can reach 99%. This performance is reached with reduced absorbent consumptions since it is then possible to operate slightly above the stoichiometry. The drawback of this option is a much higher investment cost than in the case of dry processes and, in practice, only very high-capacity units are equipped with this type of plant.
The objective of the present invention is to approach the performances of wet processes while carrying on with dry processes. To increase the rate of collection of acid pollutants, two solutions are possible a priori:on the one hand, to increase the gliding between the gas and the particles, but then the laws of mechanics seem difficult to bypass; on the other hand, to achieve massive recycling of the absorbent without using sophisticated and expensive gas-solid separation means. This second option is the basis of the present invention.